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Sensors 2013, 13, 17222-17240; doi:10.3390/s131217222 OPEN ACCESS sensors ISSN 1424-8220 www.mdpi.com/journal/sensors Article New Methodology of Designing Inexpensive Hybrid Control-Acquisition Systems for Mechatronic Constructions Jacek Augustyn Faculty of Electrical Engineering, Automatics, Computer Science and Biomedical Engineering, AGH University of Science and Technology, al. Mickiewicza 30, Krakow 30-059, Poland; E-Mail: [email protected]; Tel.: +48-126-174-014; Fax: +48-126-341-568 Received: 11 October 2013; in revised form: 25 November 2013 / Accepted: 26 November 2013 / Published: 13 December 2013 Abstract: This article presents a new methodology for designing a hybrid control and acquisition system consisting of a 32-bit SoC microsystem connected via a direct Universal Serial Bus (USB) with a standard commercial off-the-shelf (COTS) component running the Android operating system. It is proposed to utilize it avoiding the use of an additional converter. An Android-based component was chosen to explore the potential for a mobile, compact and energy efficient solution with easy to build user interfaces and easy wireless integration with other computer systems. This paper presents results of practical implementation and analysis of experimental real-time performance. It covers closed control loop time between the sensor/actuator module and the Android operating system as well as the real-time sensor data stream within such a system. Some optimisations are proposed and their influence on real-time performance was investigated. The proposed methodology is intended for acquisition and control of mechatronic systems, especially mobile robots. It can be used in a wide range of control applications as well as embedded acquisition-recording devices, including energy quality measurements, smart-grids and medicine. It is demonstrated that the proposed methodology can be employed without developing specific device drivers. The latency achieved was less than 0.5 ms and the sensor data stream throughput was on the order of 750 KB/s (compared to 3 ms latency and 300 KB/s in traditional solutions). Keywords: sensor; real-time; control system; embedded system; USB; Android; microcontroller Sensors 2013, 13 17223 1. Introduction Advanced control, acquisition-recording and automation systems require extensive communication interfaces to integrate with external computer systems and highly ergonomic sophisticated user interfaces. Processing of user data is normally performed by either embedded solutions (microprocessors) or personal computers equipped with I/O cards. This approach requires specialised software and dedicated hardware drivers. Widely popular mobile devices, such as phones, tablets and personal digital assistants offer an alternative. Mass production makes them increasingly affordable and attractive as a component of acquisition, control and recording applications. These devices typically comprise several modules, including colour touch screens and a range of communication modules, such as Wi-Fi, Bluetooth and GSM/3G. They are compact, small and relatively resistant to shocks. Large amounts of data can be stored using Secure Digital (SD) cards while Universal Serial Bus (USB) interfaces allow external devices such as pen-drives and keyboards to be attached. Cameras have become a standard in these devices, but many also feature GPS modules or other sensor types such as compasses, accelerometers, etc. The devices are battery-powered and energy efficient. Mobile devices run their on-board operating systems with graphic libraries and user interface and also handle communications. Android has a special place on the operating system market due to its openness, which means that programmers can use free libraries. Building of an acquisition and control device from separate components would take much more time and expense than when using ready commercial off-the-shelf (COTS) components. Separate components are microprocessors, display panels, communication modules, operating systems, including their programming and testing. The COTS approach, however, faces two fundamental problems: operating system response time and latency of external communication. COTS designs are equipped with operating systems intended for general purposes but not optimised for short response times of single milliseconds or less. Such short times are required for the target applications considered in this article. Control and acquisition devices also rely heavily on the integration with external sensors, converters and actuators that require highly reliable communication with low-latency and, in most cases, large data streams. COTS devices tend to be closed hardware designs where access to classical Recommended Standard 232 (RS232) or Serial Peripheral Interface (SPI) ports can be very difficult or outright impossible and integration with sensors and actuators highly problematic. Most current COTS hardware is equipped with a host class USB interface [1] intended for external pen-drive memories, keyboards or mice. USB links can achieve high data exchange speeds, but are much harder to programme than SPI and RS232 interfaces. This paper proposes the new, hybrid architecture of acquisition, control and recording solutions involving Android-running COTS devices module (e.g., tablets or mobile phones). The sensor and actuator module is based on a single chip microcontroller, capable of short response times, equipped with analogue-to-digital (AD), pulse-width modulation, digital-to-analogue converters and digital I/Os. These two modules are proposed to be integrated with each other using a new concept involving USB with standard operating system (OS) components rather than developing dedicated hardware drivers. The presented system is targeted to acquire sensor data (from encoders, accelerometers, magnetometers, AD) installed on mechatronic constructions, handle relatively large sensor data Sensors 2013, 13 17224 streams and provide real-time supervisory control (predictive controller, linear-quadratic controller) with very low latency. The described architecture is characterised by low cost and hardware simplicity through the use of standard 32-bit processors with an ARM/CortexM core with a built-in USB port. The proposed architecture also has a wide range of potential applications. Such a system can be a framework of embedded acquisition-recording devices, including energy quality measurements, smart-grids and medicine. Due to the cooperation of many software modules and interactions with operating system, there is a need to conduct an experimental real-time performance study. The main contributions of the paper include: A novel hybrid hardware architecture for acquisition, control and recording devices involving Android-running COTS components with a direct USB link which allows one to avoid additional serial-USB converters; A novel software architecture and the idea of using standard system libraries which allow one to avoid implementing dedicated hardware drivers for the USB link; Detailed results of experimental research on the real-time performance of the proposed hybrid system which are important for closed-loop control applications such as robot controls. Detailed real-time throughputs results are important for acquisition and recording applications. The results of statistical analyses of various size packets are presented; Proposed optimisation methods and their experimentally measured results. 2. Related Work The general concept of hybrid acquisition-control systems using mobile COTS devices can be found in [2]. The use of COTS-based hardware solutions is widely discussed and advocated in [3] while some application designing methodologies for demanding applications are described in [4]. COTS solutions have been suggested even for military [5] and marine [6] applications. The selection of an Android-based design and some devices is justified in [7,8]. Detailed analysis of energy consumption of platforms based on the ARM/CortexA-core used in mobile applications can be found in [9]. Generally, these hardware platforms are highly energy efficient, for example the OMAP™ processor and Android OS implementations consume in the range of 1.5–2 W. The professional literature suggests SPI as the interface of choice for the sensor and actuator module [10] and these ports are present in microprocessors with the ARM/CortexA core. In a COTS-based solution, however, this might require an intervention in the electronics and make the design overly complicated. Other solutions for integration suggested by researchers have included designs based on an SD port and dedicated hardware with the built-in Secure Digital Card Input/Output standard [11,12]. These examples were demonstrated to convert data for a relatively slow ZigBee network with throughput on the order of 20 Kbit/s. A similar idea applied in medical equipment known as a body sensor is presented in [13]. SD-based solutions tend to have limited applications. Such placement of the acquisition module in an SD port not only eliminates the option to log large amounts of data on SD cards, but it can also be even physically impossible to implement in some devices where MicroSD ports are placed inside. Another study explores the potential of an audio interface using Dual Tone Multi Frequency (DTMF) tones to carry information about the status of sensors and to set actuator outputs [14]. The Sensors 2013, 13 17225 connection can be wired or wireless using a built-in microphone and a loudspeaker. This approach suffers from

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